The present invention relates to current sources, and more particularly, to a current source that operates at a low supply voltage and with quasi-null current variation in relation to a supply voltage.
Current sources that operate at a low supply voltage and with quasi-null current variation in relation to a supply voltage are used, in particular, for polarizing circuits such as operational amplifiers, for example. These circuits are intended to operate over wide voltage ranges.
For example, one can consider portable devices that may be supplied either from a battery or from a main power supply. These devices can be radio devices, and devices for reading or sound reproduction. When these devices operate on a battery, the supply voltage is relatively low, on the order of 3 volts for example, and diminishes when the battery drains down to about 2 volts or less. When these devices operate from a main power supply, the supply voltage is on the order of 5 volts. There can be a ratio of 2 or even 3 between the two supply voltages.
At present the current sources used in this type of application are such as that shown in FIG. 1. This source of current, produced in this example using bipolar technology, is connected between two supply terminals. Terminal 20 is connected to a high potential VCC and the other terminal 21 is connected to a low potential Vee, which is generally ground.
The current source comprises a core C and a current mirror M mounted in series between the two supply terminals 20, 21. The core C is the part of the current source which controls an equation corresponding to the source current. In this case, it concerns a so-called VBE/R source. The core C comprises a transistor Q1, a resistance R for setting the current and possibly an additional transistor Q2. The core C is connected to one of the supply terminals 21, in this case the terminal 21 at the potential Vee. The transistors Q1 and Q2 of the core are of the same type, in this case of the n-p-n type.
In the description below, a voltage VBE represents a base-emitter voltage and a voltage VCE represents a collector-emitter voltage. The current mirror M comprises a pilot transistor Q5 and at least one recopy transistor Q4. The mirror M is linked to the other supply terminal 20, in this example, the potential VCC. The mirror transistors Q4, Q5 are of the same type, in this case of the p-n-p type, and are complementary to those of the core C. They are produced at the same time and are thus identical.
The transistor Q1 is connected between the supply terminal 21 and the recopy transistor Q4 of the mirror M. These two transistors Q1, Q4 form a slave branch 22 between the two supply terminals 20, 21. The base of the transistor Q1 is connected to a first end of the resistance R for current setting. The second end of the resistance R is connected to the supply terminal 21 at the potential Vee. The first end of the resistance R is also connected to the pilot transistor Q5 of the mirror M via the additional transistor Q2. The resistance R for setting the current, the additional transistor Q2 and the pilot transistor Q5 form a pilot branch 23 between the two supply terminals 20, 21. The transistor Q1 is configured as a diode, that is, its base is connected to its collector via the additional transistor Q2. The mirror M is connected to the other supply terminal 20, in this case at the potential VCC.
The recopy transistor Q4 of the mirror M has its emitter connected to the supply terminal 20 at the potential VCC, its collector connected to the transistor O1 of the core C and its base connected to the base of the pilot transistor Q5 of the mirror M. The pilot transistor Q5 of the mirror M has its base connected to the base of the recopy transistor Q4 of the mirror M and to its collector. It is configured as a diode. Its connector is also linked to the resistance R of the core C via the additional transistor Q2. The emitter of the pilot transistor Q5 is connected to the supply terminal 20 at the potential VCC.
The biasing current of the source is accessible at the level of the collector of an output transistor Q6, which is configured as a recopy transistor relative to the mirror M. Its emitter is connected to the supply terminal 20 at the potential VCC, and its base to the base of the pilot transistor Q5 of the mirror M. The output transistor Q6 is identical to the pilot transistor Q5. This biasing source is described on page 324 of the work xe2x80x9cAnalysis and Design of Analog Integrated Circuitsxe2x80x9d by P R GRAY and R. G. MEYER, 3rd Edition.
One can assume that in the core C, the current I crossing the resistance R, and which corresponds to the collector current of the transistor Q2, is the same as that circulating in the branch 22 by current mirror effect. Thus, one has:
I=(VT/Rxc3x971n(I/IS) 
where the thermal voltage VT equals kT/q, k is Boltzmann constant, T the temperature in degrees Kelvin and q the charge of the electron. IS represents the saturation current of the transistor Q2.
If I is known, this makes it possible to determine the expression of the polarization current Ic(Q6) of the source at the level of the output transistor Q6:
Ic(Q6)=Ixc3x97(1+VCE(Q6)/VEA(Q6)/1+VCE(Q5)/VEA(Q5)) 
where VEA(Q6) and VEA(Q5) are respectively the Early voltages of the transistors Q6 and Q5. They are equal, since the transistors Q6 and Q5 are of the same p-n-p type and are identical. The voltage VCE(Q5) is equal to VBE(Q5) because the pilot transistor Q5 is configured as a diode. The voltage VBE(Q5) remains relatively constant while VCC varies.
The current Ic(Q6) varies in the same direction as the potential difference between the two supply terminals 20, 21 since VCE(Q6) varies in the same direction as this potential difference. In the rest of the description below, this potential difference is assimilated to VCC since it has already been assumed that the supply terminal 21 is at a ground potential.
To obtain a biasing current in the opposite direction from the current Ic(Q6), that is, complementary to the current Ic(Q6), one can add a second output transistor Q3 configured as a current mirror with the Q1 transistor of the core. In this second mirror, the transistor Q1 is the pilot transistor and the transistor Q3 is a recopy transistor.
This recopy transistor Q3 has its base connected to the base of the transistor Q1, its emitter connected to the first supply terminal 21 at the potential Vee and its collector forms another source output. The collector current of the transistor Q3 is given by:
Ic(Q3)=Ixc3x97(1+VCE(Q3)/VEA(Q3))/1+VCE(Q1)/VEA(Q1)) 
Ic(Q3)=Ixc3x97(1+VCE(Q3)/VEA(Q3))/1+VBE(Q1)+VBE (Q2))/VEA(Q1)) 
VEA(Q3) and VEA(Q1) are Early voltages of the Q3 and Q1 transistors respectively. They are equal and correspond to the Early voltages of n-p-n transistors since Q1 and Q3 are identical n-p-n transistors. In this case again VBE(QL) and VBE(Q2) remain relatively constant while VCC varies, but VCE(Q3) varies in the same direction as VCC, and thus IC(Q3) varies in the same direction as VCC.
The properties of electronic circuits biased by a current source are intrinsically linked with the current consumption of their components. For example, the gain of a transistor increases as the current passing therethrough increases. To have properties as constant as possible to control electronic circuits, the biasing current should be as constant as possible regardless of the value of the supply voltage.
The biasing current source of FIG. 1 is not completely satisfactory from this point of view. In addition, this biasing current source only starts up when the supply voltage Vcc reaches a relatively high value. This property is disadvantageous when the supply voltage is provided by a battery which is somewhat discharged, since there is the risk that the biasing current may not start up.
The minimum supply voltage for starting up the current source is given by:
VCCmin=VBE(Q1)+VBE(Q2)+VCEsat(Q4) 
that is, 2VBE+VCEsat. This equation applies to branch 22. For branch 23:
VCCmin=RI+VCEsat(Q2)+VBE(Q5) 
VCCmin=VBE(Q1)+VCEsat(Q2)+VBE(Q5) 
that is, VCCmin=2VBE+VCEsat. This voltage VCCmin is on the order of 1.7 volts with bipolar transistors.
In view of the foregoing background, an object of the present invention is to overcome the disadvantages presented by the current source illustrated in FIG. 1.
The present invention relates to a current source whose current is almost constant while the supply voltage varies and which, in addition, can start up at a low supply voltage.
More precisely, the present invention relates to a source of current set between two supply terminals. The current source comprises a current mirror and a core connected together. These items are discrete. The current mirror and the core form several branches to be connected between the two supply terminals. The mirror comprises a pilot transistor and at least one recopy transistor. The core comprises a first transistor, a second transistor, and a resistance.
The first core transistor and the first recopy transistor are connected together to form the first branch. The resistance and a second recopy transistor of the mirror are linked together to form the second branch. The pilot transistor and the second core transistor are linked together to form the third branch. The first transistor of the core is connected to the second branch between the resistance and the second recopy transistor. The second core transistor is connected to the first branch between the first core transistor and the first recopy transistor.
An output transistor makes the source current accessible. This transistor is a supplementary recopy transistor of the mirror, but is placed off-branch. The mirror transistors are of the same type, and the same applies to the core transistors. In addition, the core transistors and the mirror transistors are complementary.
The mirror transistors may be bipolar. To compensate for the base currents of the mirror transistors, the pilot transistor of the mirror may be configured as a diode through a supplementary transistor. The mirror transistors may be MOS transistors. In the same way, the core transistors may be bipolar transistors or MOS transistors. The supplementary transistor may be either a bipolar or a MOS transistor.